1. Field of the Invention
This invention relates to a process for forming an optical film structure for an optical waveguide and the optical waveguide by means of Flame Hydrolysis Deposition (FHD).
2. Related Background Art
FIGS. 1A-1C show fabricating steps of a conventional process for fabricating a film structure for an optical waveguide having under and over cladding layers and a core surrounded by them. In the step shown in FIG. 1A, a first porous vitreous layer (SiO.sub.2 +B.sub.2 O.sub.3 +P.sub.2 O.sub.5) to be the under cladding layer 20 is made of SiCl.sub.4, BCl.sub.3 and POCl.sub.3 on a substrate. In the step shown in FIG. 1B, a second porous vitreous layer (SiO.sub.2 +GeO.sub.2 +B.sub.2 O.sub.3 +P.sub.2 O.sub.5) to be the core layer 30 is formed of SiCl.sub.4, GeCl.sub.4, BCl.sub.3 and POCl.sub.3 fed from a burner, on the substrate. In the step of FIG. 1C, all the porous vitreous layer are sintered to be transparent. Subsequently the transparent core layer 30 is patterned as required, and the over cladding layer (not shown) is formed thereon.
But, in this process for forming optical waveguide films, when all the porous vitreous layers are sintered, the GeO.sub.2 component of the porous vitreous layer to be the core layer 30, i.e., core soot, adversely evaporates into the under cladding layer 20. This results in a problem that the GeO.sub.2 component which has evaporated downward diffuses into the under cladding layer 20, and the interface between the core layer 30 and the cladding layer cannot be accurately controlled. FIG. 1C shows such state.